Electric motors can exhibit a high torque output from very low revolutions per minute (RPM). Internal combustion engines, have very low torque at low RPMs, their torque increasing with increasing RPM to peak at a maximum, usually above 1000 RPM. However, the high torque of electrical motors cannot be utilised efficiently since the high torque will cause the driven wheel of the car to skid or slide. The maximum possible acceleration of a car on wheels is limited by the laws of physics, specifically, the coefficient of friction.
The coefficient of friction between two surfaces has two distinct parts: the coefficient of sliding friction (also known as just coefficient of friction), and the coefficient of starting friction (also known as the coefficient of static friction). For ease of discussion, the coefficient of sliding friction can be designated as Cslide and the coefficient of starting friction as Cstart. The coefficient of sliding friction, Cslide, defines the force required to keep an object sliding on a surface, specifically, F=(W)X(Cslide), where F is the force required to keep an object of weight W sliding on a surface which has a Cslide (coefficient of sliding friction) for the two materials which compose the object and the surface on which it is sliding. Cslide is dependent on the two materials and is independent of moderate speeds, although it usually decreases slightly above 30 to 40 feet per second. Cslide is less than 1.0 and is always lower than Cstart for the same object on the same surface, i.e. for any given material on any given surface, Cstart>Cslide.
The coefficient of starting friction, Cstart, refers to the force required to cause an object at rest to begin sliding on a surface. The required force to start an object sliding is: F=(Cstart)X(W). Cstart is greater than Cslide, so once the object begins to slide, it requires less force keep the object sliding.
The acceleration imparted on a car is limited by the coefficient of friction, i.e., A=F/M, where: A=acceleration, F=the force applied to the car, and M=the mass of the car. Since the force for acceleration F is limited by the coefficient of friction, thus the acceleration is limited by Cslide and Cstart.
A sliding wheel can impart a forward force on the car equal to the force due to the coefficient of sliding friction, i.e., F=(W)X(Cslide), where F is the imparted force of acceleration, Cslide is the coefficient of sliding friction between the tire of the driven wheel and the road, (which varies considerable with the type of road surface, and conditions such as temperature, wetness, etc.), and W is the combined total weight of the tire onto the road surface.
If the wheel is not skidding, then the forward force of acceleration can be as high as F=(W)X(Cstart). Since Cstart>Cslide, the possible acceleration is greater as long as the wheel does not skid or slide. Thus the traction of a tire on the road is significantly higher when the surface of the tire is at rest relative to the surface of the road, as opposed to when the surface of the tire is sliding or skidding relative to the surface of the road. This does not mean that the tire is not moving; in fact, the tire may be travelling at a great speed, but if the tire is rotating at the correct rate, the bottom surface of the tire will match the speed at which the surface of the road meets the tire; that is, the tire is rolling on the road. All that matters is that the two surfaces of the tire and the road are momentarily at rest with respect to each other, where the two surfaces meet. The traction in that case is thus limited by Cstart.
If the two surfaces of the tire and road are moving relative to one another, then the traction is limited by Cslide. Since Cstart>Cslide, the traction in the first case greatly exceeds the second case. It is exactly this principle which is the basis for many anti-lock braking systems (“ABS”), which lessen the braking action when wheel skid is detected, allowing the tire to freewheel, and to re-establish zero relative speed and thus provide conditions for Cstart.
Previous traction techniques for vehicles having internal combustion engines or electric motors have been limited in ability to apply torque to drive wheels under various road conditions and with optimal energy efficiency.